Fortunately, blockchain as a ledger technology [1]} can provide attractive solutions for FL-based intelligent edge computing blackdue to its unique features such as decentralization, immutability, and traceability. Indeed, a blockchain is a form of blocks linked by hash values under the control of a consensus mechanism, e.g., Proof-of-Work (PoW), enabled by miners which mine blocks with digital signature to make linked blocks immutable against modifications and alterations [1]}. blackBy using blockchain, FL can be implemented via decentralized data ledgers without requiring any central server which mitigates risks of single-point failures, and any update events and user behaviours are traced by all network entities in a transparent manner. Furthermore, one can easily trace the origin where a model parameter is modified or updated during the training process through transaction logs, which cannot be satisfied in traditional FL systems. Particularly, the integration of FL and blockchain creates a new paradigm called FLchain, which potentially
transforms intelligent edge networks with decentralized and secure natures [3]}, [4]}. In FLchain, each device can act as a client with equal rights to update and aggregate the learning model in a decentralized manner. More specifically, each client first initializes a data model and computes learning parameters. Then, it uploads the computed update to a group of miners in a form of transactions. Here, miners combine transactions (which contain local updates) created by clients into a block after a period of time, which is then verified by miners through a mining process. Once the block is mined, it is appended to the blockchain and broadcast to the entire network. Each client downloads the block, which contains the updates of other clients, and computes a new version of the global model. The process iterates until the global loss function converges or the desired accuracy is achieved. In this way, blockchain helps eliminate the need for a central server which thus potentially alleviates communication costs and achieves better intelligent network scalability. Moreover, blockchain provides high security for FL training through immutable block ledgers.
